One Per Cent: Artificial muscle for soft robots can bend in sunlight: When hit with UV light at a wavelength of 365 nanometres it expands, bending and increasing in mass by taking on water. Visible light at a wavelength of 430 nm restores the muscle to its previous form. "The gel absorbs water like an expanding and contracting sponge," says Harada.
The muscles work via the interaction of two chemical compounds in the gel - azobendrine and cyclodextrin - which react differently under different light. The direction in which the gel bends can then be controlled by shining UV and visible light from various angles.